Abstract: The study shows that the distribution of Oncomelunia hupensis is closely related with theunderground water level in forest land. O.hupensis has a maximum density,when the underground water level is 20～40 cm, and approaches to zero when the level is<1.55 cmor >75.7 cm. Its density in forest land is also affected by the flooding period of Changjiang River. Within a period of 100 inundated days, the longer the inundated time,the greater the O.hupensis density. Among 36 species of grass in herb layer, Lecnurks heterophgllas, Equisetum arvellse, Calystegia hederacea, Polygonum lapathifolium and Astragalas sinicus are negatively, and Roedria scandens and Oenanthe javanica are positively correlated with the density of O.hupensis.

Key words: Underground water level, Inundated time, Grass in herb layer, Oncomelunia hupensis, nitrogen addition, particulate organic carbon, water-soluble organic carbon, phosphorus addition